1. Field of Invention
The present invention relates generally to the field of dyeing textiles. More specifically, the present invention is related to using vat dyes in a continuous process to produce a variety of colors during the production of denim.
2. Discussion of Prior Art
“The older, the better look” has been the philosophy of the blue jean industry. Blue jeans are dark blue when they are first produced. As they are worn and washed, the abraded places become a different color than the rest. Today there are numerous techniques to produce this natural washout look in denim. Some of these techniques include stone washing, enzyme washing, bleaching, acid washing, resin treatment, ozone washing, neutralization, tinting and garment dyeing, local tinting, chemical spraying, local bleaching, sand blasting, brushing, laser, moustache or whiskers, damage and cutting. However, this natural worn-out or washout effect only happens with fabric produced with yarn that has ring effect dyeing (i.e., perimeter dyeing), or specifically in blue jean denim, dyed with Indigoid vat dye. Ring effect in yarn is defined when dye does not penetrate into the yarn and only perimeter/external dyeing is achieved.
Today, the production of yarns with ring effect dyeing is feasible only through a few continuous processes. The most popular continuous processes used for dyeing yarns are rope (long chain) dyeing, slasher (sheet) dyeing, and loop dyeing (looptex). In these processes, indigo-derived vat dye is added in reduced form or in mixture with reducing agents to a dye tank. FIGS. 1-4 illustrate prior art for continuous dyeing cotton yarns or fabric.
Indigo is a water insoluble organic substance that can be reduced to a water-soluble form and used to dye yarns or fabrics. Following dyeing, the dye is oxidized, which returns the dye to its water insoluble form on the yarn. Indigoid vat dyes have been primarily used for cotton yarns, which have given rise to the popularity of denim fabric today.
It is known, in the continuous dyeing of yarns, to add the dye in the form of a concentrated stock vat. FIGS. 1 and 2 illustrate 2-16 dip-dye tanks equipped with squeezing/skying apparatus used for applying the vat dyes. A minimum of 2 dip-dye tanks is required for rope and slasher dyeing. Squeezing and skying takes place between the dipping steps and the dye is oxidized by air passage. To avoid dye depletion of the dip vats, the dye is replenished from stock vat dyes having a concentration greater than 80 g/l or concentration of at least 20% stock vat. These reduced stock vat dyes are introduced in the circulation line at the dye tank. FIG. 3 illustrates loop-dyeing process wherein direct beam is introduced to at least one dip-dye tank and squeezing/skying apparatus. The direct beam is recycled or looped several times in the same dip-dye tank. FIG. 4 shows continuous dyeing process for fabric where “ready-to-dye” fabric is added to a series of dip dye tanks squeezing/skying apparatus.
Similar to indigo, other vat dyes (indigoid or anthraquinonoid) also have excellent all-round fastness properties on cotton. However, unlike indigo, most vat dyes have high molecular weight, high substantivity, and low solubility. Hence, for most vat dyes no reduced stock solution is available in the market to use in denim fabric production. Additionally, adding reducing agents to high concentration of most vat dyes in feeder dye tank results in precipitations due to their poor solubility.
It is desired to produce textile material with ring effect dyeing using different colors, such as, but not limited to: orange, red, violet, pink, green, yellow, black, brown, blue, khaki, gray, purple, navy, beige, or other vat dye colors or combinations thereof. However, the production of textile material with ring effect dyeing has been limited to vat dyes with high solubility, limiting the color of denim fabric. Vat dyes, in particular anthraquinonoid vat dyes, have a wide range colors. However most these different color dyes have low solubility.
U.S. Pat. No. 5,518,508 (hereinafter referred to patent '508) discloses a method for continuous dyeing of yarn. Patent '508 uses dye dispersion instead of stock vat to solve the problem of supersaturation and insufficient concentrated stock vat. However, the circulating concentration of dyes is usually low (approximately 50:1 ratio from stock vat to circulating dye) which results in low reduction rate of vat dye. It is known that the vatting rate is a function of dye and reducing agent concentration. The prior art requires a high concentration of reducing agent for reduction of dyes in the circulating liquor. Increased unreduced dye in the circulating dip-dye tank results in poor dyeing and finished yarn has poor rubbing and washing fastness.
Whatever the precise merits, features, and advantages of the above cited references, and none of them achieve or fulfill the purposes of the present invention.